In a facsimile terminal, for instance, an image to be transmitted or an image received is stored in a page memory. Also, in an image editing device, an image to be edited or an image edited is similarly stored in a page memory. The required capacity of the page memory is increased as the size of the image is increased and also as the resolution of the image is enhanced. Further, when a color image is to be stored, there is required a capacity three times larger than in the case of a monochromatic image. For example, when a full four-color image of A3 size is stored at a resolution of 400 dpi (dot/inch), a memory capacity reaches as great as 128M byte. If the memory capacity increases like this, not only the cost of the page memory increases but also it takes much time to read and write image data, leading to an extended process time.
As a solution of such problems, it has been examined that an image signal is encoded highly efficiently in such a form that the image signal can be edited while it remains encoded data. Such encoding requires the following three characteristics.
Firstly, compressibility must be uniform. That is, because the page memory is constructed such that it has a limited capacity, it is necessary to be able to encode the images at a preset compressibility independent of the images. Secondly, it is necessary that the encoded data can be edited as they are. That is, in order to be able to obtain the two-dimensional position of the image signal directly from the encoded data, it is necessary that the image signal divided in a given unit can be encoded in a given amount of codes and also that encoding/decoding can be achieved independently in every given unit. Thirdly, it is necessary that an encoding/decoding processing can be performed uniformly. That is, since the image is encoded on the page memory, it is necessary that the image can be encoded and decoded at a high and constant speed.
In a conventional image encoding device for storage and transmission, it is necessary to restrict the spacial/frequency redundancy of the image signal as much as possible and, therefore the compressibility varies according to the variations of the redundancy of the respective image signals. Also, there has been shown a tendency to introduce a higher level of encoding processing, which makes it difficult to execute the encoding/decoding processing independently in every given image division unit. Further, due to introduction of an adaptive processing, a quantity of operation necessary for the encoding/decoding processing varies greatly according to the variations of redundancy in each of the image signals. For these and other reasons, with such a conventional image encoding device, it is difficult to satisfy the above-mentioned characteristics.
An example of such efficient encoding methods has been proposed as a so-called BTC (Block Truncation Coding) scheme in Published Unexamined Japanese Patent Application No. Sho-57-174984, in which image signals are divided into block units each of a certain size and the shapes of every block is to be truncated.
The outline of the BTC scheme will be described with reference to FIGS. 16 and 17. In this scheme, an image as shown in FIG. 16 is divided into a plurality of block units each being composed of L.sub.i .times.L.sub.j picture elements as shown in FIG. 17. Assuming that L=L.sub.i =L.sub.j and tone of the picture elements in a block are respectively expressed as a.sub.ij, then the average tone P.sub.0 of the whole of the block is P.sub.0 =.SIGMA.a.sub.ij /L.sup.2. As shown in FIG. 18, if the average tone and the number of picture elements respectively having lower tone than the average tone P.sub.0 in the block are P.sub.1 and N.sub.1, respectively, then the average tone P.sub.1 and the number N.sub.1 are expressed by: ##EQU1##
If the average tone and the number of picture elements respectively having higher density than the average tone P.sub.0 in the block are P.sub.2 and the number N.sub.2 are expressed by: ##EQU2##
These expressions are established under the condition of .PHI..sub.ij =0 if a.sub.ij .ltoreq.P.sub.0, and .PHI..sub.ij =1 if a.sub.ij &gt;P.sub.0.
Now, in case of inserting integers m and n smaller than L.sup.2 and the number of tone levels of tone, it is discriminated as the tone distribution density in the block being uniform that .vertline.P.sub.1 -P.sub.2 .vertline.&lt;m, or N.sub.1 &lt;n, or N.sub.2 &lt;n. Thus, .phi..sub.ij are all considered as 0. As shown in FIG. 19, the whole blocks are represented only by the densities P.sub.0. Also, when .vertline.P.sub.1 -P.sub.2 .vertline..gtoreq.m and N.sub.1 .gtoreq.n and N.sub.2 .gtoreq.n, then the tone distribution density in the block is considered as being not uniform and, as shown in FIG. 20, the blocks are represented by both average tone P.sub.1 and P.sub.2. In this case, P.sub.1 is positioned where .phi..sub.ij is 0, and P.sub.2 is positioned where .phi..sub.ij is 1, correspondingly. .phi..sub.ij represents the information related to the shape of the block, referred to as resolution information, while P.sub.0 or P.sub.1, P.sub.2 represent the information which indicate the tone levels, referred to as gradation information. When encoding the resolution information, the resolution information is encoded every several pair of lines by an ordinary binary coding scheme, and the successive length of blocks equal to each other in the information values is also encoded by a well known run length scheme to be transmitted. Referring to the parameters m, n employed in this encoding method, m serves also as a discrimination of threshold to eliminate an isolated noise of the image and n serves also as a discrimination of threshold to eliminate the slight fluctuations of tone in the blocks, so that the image is to be smoothed so much as both the parameters m, n have larger values.
In this scheme, however, since picture elements in a block can be expressed only by two specified tone levels at maximum, there has been a issue that if block size L is made larger to obtain more high compression, the image tone levels becomes large, and particularly the distortion is not negligible in the area having a smooth tone gradation. In addition there is much redundancy, since the resolution information allocated uniformly to all blocks. Although in order to address this issue, it has been tried to reduce the redundancy by means of the binary coding of the resolution information every several lines, it has not been satisfactory.
In addition, there has been a problem that it is difficult to control the coding rate by selecting the parameters m and n, and it is also difficult to edit an image as it is in an encoded form.